2. Ring morphologies

2.1. Procedure

The data base contains some ring-shaped objects of the AM catalogue, several ring-shaped galaxies of Few & Madore (1986), FAHR, and Faúndez-Abans et al. (1994), and of the atlas of polar-ring galaxies of Whitmore et al. (1990).

The selected galaxies have been inspected on the J and R SRC/ESO Survey film copies to confirm their morphological types and, when possible, to make measurements of the bulge and ring diameters with a micrometer microscope. After careful inspection of the ring and bulge structures, they have been classified by their visual appearance (see Appendix A with Notes on the objects, available in electronic form only, at the Centre de Données astronomiques de Strasbourg - CDS). No photometric calibrations of the plate copies have been made; therefore, the measurements of the bulge's and ring's minor and major axes have not been made at the same isophotal level, but judiciously by eye, instead. This procedure often yields low-error dimensions for the ring, except in those cases when tidal distortions are large; on the other hand, the bulge's ellipticity is subject to the plate depth and influence of the disk and projected angular distance to the ring itself.

In a first step, when all morphological details had been taken into account, the galaxies were grouped in 29 different types of objects. Those which are NRG were then excluded from this classification. Next, the several morphological categories have been compressed into five, following the general behavior of the ring. Table 1 (click here) displays these five peculiar ring families.

 

Families of Rings Code Basic Structures

Polar P (a) Spindle (b) Saturn (c) Worm-like

Hoag HL (a) Hoag (b) Hoag-like

Elliptical E (a) Knotted

(the warped behavior (b) Smooth

is often present) (c) "Solitaire''

Irregular I

Centrally Smooth CS

 

2.2. Types of rings

The candidate objects display a wide variety of ring and bulge morphologies. Some structures could be remnants of interaction: rings with off-center bulge, external and internal rings with twisted principal axes, and warped rings. Others are in fact interacting ring galaxies showing bridges, filaments, crisps, plumes, and warped rings. It is not unusual to find knotty, smooth, polar, and elliptical rings.

1)

Polar rings

It is generally accepted that the observed structure of polar ring galaxies come from accretion of matter from a nearby galaxy or by merging of a gas-rich companion. A study of the polar NGC 4650A by Sackett & Sparke (1990) suggests that the central condensation is much smaller and less massive than the ring. They propose that a gas-rich dwarf galaxy may have disintegrated to form the ring instead of the gravitational stripping by/of a neighboring galaxy, as had been proposed earlier. This kind of object has been very well discussed by Whitmore et al. (1990), who present a photographic atlas and describe its appearance from a variety of viewing angles. They also report photometric and spectroscopic observations of several galaxies. A short and beautiful review can be found in Lucas (1993). For recent works on polar ring galaxies, the reader is referred to Reshetnikov (1997), Reshetnikov et al. (1995, 1996, and 1997), Hagen-Thorn & Reshetnikov (1997) and references quoted therein.

There are three basic structures that can be associated to this family:

(a)

Spindle. The traditional polar ring galaxies. These objects appear to have spindle-shaped bulges (the S0 galaxy) with a ring nearly perpendicular to the galactic major axis (polar ring).They can be further subdivided into two basic structures: (i) Objects with a well-defined ring without or with very faint external material, archetypes being ESO 415-G26 and HRG 19502, and   (ii) objects which show rings with prominent outer extended material; archetypes are A0136-0801, AM 0226-320. The reader may also refer to the catalogue of box- and spindle-shaped objects by Jarvis (1986).

(b)

Saturn. These objects have a spherical bulge with a bright round ring. It is proposed to also classify as saturn-type those galaxies of which bulge is a little elliptical. It is very likely that the saturn-like objects are only an effect of viewing angle, as displayed in Fig. 1 of Whitmore et al. (1990) for polar ring galaxies. Archetype is HRG 54103.

(c)

Worm-like. A long worm-like bulge is the main characteristics. Whitmore et al. (1990) suggest that they are possible candidates for polar-ring galaxies. The ring around the bulge generally shows one knot with bulge-like dimensions, just as the "solitaire'' ring galaxies (see below). Archetypes are ESO 199-IG12 and HRG 32501.

2)

Hoag rings

These objects are characterized by a circular ring around the bulge. There are two kinds of objects in this family:

(a)

Hoag. The name comes from Hoag (1950), who discovered an object which he described as a "perfect halo'' surrounding a small diffuse bulge. At a first glance, these objects appear to be planetary nebulae because of the circular symmetry and the dimensions of the bulge (see also Brosch 1985, 1987).

(b)

Hoag-like. These objects have Hoag-like morphology with the following differences:  (i) some of them have low ellipticity bulges (Archetype FM 59-12),  (ii) others, a comparatively large spherical bulge, archetypes FM 327-23 and FM 235-17. It is possible that some spindle-likes and saturns, when seen almost end-on, would appear as Hoag or Hoag-like.

3)

Elliptical rings

Some of these objects are classified as  RN  by Theys & Spiegel (1976), and as  P  objects by  FM. In general, they show an off-center bulge, but in a few cases it is possible to discern a centered bulge with an elliptical ring, possibly inclined    to   to the line-of-sight. The ellipticals is a large family, where warped rings are often present. Three basic structures can be discerned:

(a)

Knotted. The objects in this category have knotted rings and may be evidence of ring deformation as a result of tidal interaction; they may be undergoing star formation. In some cases, an internal elliptical ring is also present. Examples of these objects are HRG 35002, HRG 03401, and AM 2134-471.

(b)

Smooth. They have either centered or off-centered bulge. Clear evidence of collision can be seen in some of the latter. Examples of these objects are: HRG 13801, which also shows a bridge of material extending within the intruder, and HRG 08901, which is also interacting but displays no visual evidence of a bridge.

(c)

"Solitaire''. The bulge is on the ring, resembling a one-diamond finger ring called "solitaire'' ("single-knotted ring"). The ring generally looks smooth and thin on the opposite side of the bulge, but there are a few cases where the ring appears broken on this opposite side. A few galaxies show a filamentary ring structure. The intruders may be elliptical to various degrees. Archetypes are FM 251-47 and FM 188-15.

4)

Irregular

Several galaxies show irregular distortions in their structures which resemble pseudo-rings, arms, and curved tails.

5)

Centrally smooth rings

These objects are knotted ring-like structures without an apparent bulge. The term has been originally introduced by Appleton & Struck-Marcell (1996) and have been adopted in this work. Recently, Appleton et al. (1996, see also references quoted therein) have studied the northern archetype VII Zw 466 and found evidence of tail interaction, where one companion galaxy points to the intruder, which must have collided with VII Zw 466. A southern archetype is HRG 50201.